The present invention relates to methods for the dimensionally accurate photolithographic transfer of sub-.mu.m structures through a bilayer technique.
In the photolithic production of resist structures, the resolution, i.e., the smallest possible structure that can be imaged (critical dimension=CD), is defined by a number of parameters. These parameters are the photolithographic properties of the resist, the wavelength (.gamma.) of the light used for the exposure, the numerical aperture (NA) of the imaging optics, and the reflective behavior of the light at phase boundaries between two materials having different optical density.
As demonstrated by the following equations, that are dependent on one another, a high structure resolution can be achieved by using a shorter wavelength or a larger numerical aperture. However, on the other hand, focal depth decreases by an even greater degree, so that the advantages of using radiation having a short wavelength can only be achieved in extremely thin resist films having a high uniform thickness. ##EQU1## In the above equations, parameters f.sub.1 and f.sub.2 are factors that are associated with the method and system. These parameters only slightly vary given the systems that are presently optimized. In the above equations, DOF references the depth of focus.
Chemical factors determine the quality, the uniformity of the distribution, and the penetration depth of the (chemical action) of the light in the resist. The particular chemical factors include the solubility differences between irradiated and non-irradiated regions during development with gaseous or liquid agents. With respect to the selection of liquid developing agents, it is desirable that the agent is selected such that there is a low capacity for swelling of the resist, as well as, a high selectivity between irradiated and non-irradiated resist regions.
A high resolution can be achieved using positive photoresists that, among other things, are distinguished by a low intrinsic absorption of the basic polymer at the irradiation wavelength and by a good bleaching behavior of photo-active constituents during irradiation.
By varying such parameters as irradiation duration and the developing process, further variations of the photostructuring method are possible. It is also possible to attempt to optimize the resolution of a resist by varying the type of developer and the development conditions.
Although the above variations are possible in an attempt to optimize the resolution of the resist, even given a resist that optimally resolves in a uniformly thin layer on a planar substrate in single-layer technique, the resolution is drastically reduced when the resist is applied onto substrate having substrate steps and surfaces that reflect differently. Such reflections lead to exposure in undesired regions and, thus, generates an image that is not sharp.
The use of a bilayer technique can avoid these problems. In a bilayer technique, the photoresist to be structured is applied in a thin layer as a top resist over a lower, planarizing, first resist layer (a bottom resist). The top resist structure is produced in a normal manner and is ultimately transferred into the bottom resist structure in an anisotropic etching process; for example, in an oxygen/RIE plasma, wherein the top resist structure functions as an etching mask.
Because of the isotropic portion, that cannot be avoided in the etching step that otherwise acts predominantly anisotropically, an under-etching of the bottom resist structures occurs. This results in side walls having a concave structure.
It is known, in some bilayer methods, to use silicon-containing top resists having about 10 weight percent of silicon. During the development and following transfer of the structure into the bottom resist, however, the transfer only results in structures that are at least 10 percent narrower than the structures prescribed on the mask.
European Patent Application No. EP A 0 136 130 discloses a method wherein a top resist structure, composed of novolak, is treated with gaseous titanium chloride in order to improve its etching resistance to an O.sub.2 --RIE etching plasma. However, because of the reaction of the top resist with the titanium chloride, that only superficially occurs, there is a lateral loss in dimension upon transfer of the top resist structures onto the bottom resist with this method. Such a lateral loss is not acceptable in sub-.mu.m structures in the range of 0.5 .mu.m. Moreover, due to an incomplete conversion of the hydrolyzable chlorine at the titanium, a later contamination of the wafer or of the apparatus with HCl in a following process step is possible.
U.S. Pat. No. 4,690,838 proposes a method for the quality enhancement of top resist structures. The top resist is treated with gaseous reactants such as hexamethyldisilazane, chlorosilanes, or aminosilanes after the development. Again, a lateral loss in dimension arises in the transfer of structure since the quality enhancement occurs only at the surface or at phase boundaries.
An article by Sezi et al, in SPIE, Vol. 811 (1987), pages 172-179, presents a detailed, critical discussion of known methods of bilayer techniques.